Boot Sequence: Difference between revisions

Boot Sequence (view source)

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→‎Master Boot Record: They are bytes 511 and 512 although the zero-based numbering is probably better - this makes it clearer

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Revision as of 12:12, 17 August 2016 (view source) Glauxosdever (talk \| contribs) (Byte indexing starts from 0) ← Older edit		Revision as of 16:56, 17 August 2016 (view source) osdev>Jnc100 m (→‎Master Boot Record: They are bytes 511 and 512 although the zero-based numbering is probably better - this makes it clearer) Newer edit →
Line 5: == Master Boot Record == The (legacy) BIOS checks bootable devices for a boot signature, a so called magic number. The boot signature is in a boot sector (sector number 0) and it contains the byte sequence 0x55, 0xAA inat ~~bytes~~byte offsets 510 and 511 respectively. When the BIOS finds such a boot sector, it is loaded into memory at <tt>0x0000:0x7c00</tt> (segment 0, address 0x7c00). (However, some BIOS' load to <tt>0x7c0:0x0000</tt> (segment 0x07c0, offset 0), which resolves to the same physical address, but can be surprising. A good practice is to enforce CS:IP at the very start of your boot sector.) Execution is then transferred to the freshly loaded boot record. On a floppy disk, all 512 bytes of the boot record may contain executable code. On a hard drive, the [[MBR (x86)\|Master Boot Record]] (MBR) holds executable code at offset 0x0000 - 0x01bd, followed by table entries for the four [[Partition Table\|primary partitions]], using sixteen bytes per entry (0x01be - 0x01fd), and the two-byte signature (0x01fe - 0x01ff).